How to put a model in a T-pose automatically?
If the armature was created (Edit Mode) in T-pose, you can easily get it back in armature tab by clicking Rest Position button and get your pose again with Pose Position button.

If you need the T-pose as a pose, select all bones (with A ) in Pose Mode and Alt + R , Alt + G, Alt + S to clear all transforms. You can then create a T-pose in the Pose Library to activate it easily.